Microbial corrosion biofilm

Many of the by-products of microbial metaboHsm, including organic acids and hydrogen sulfide, are corrosive. These materials can concentrate in the biofilm, causing accelerated metal attack. Corrosion tends to be self-limiting due to the buildup of corrosion reaction products. However, microbes can absorb some of these materials in their metaboHsm, thereby removing them from the anodic or cathodic site. The removal of reaction products, termed depolari tion stimulates further corrosion. Figure 10 shows a typical result of microbial corrosion. The surface exhibits scattered areas of localized corrosion, unrelated to flow pattern. The corrosion appears to spread in a somewhat circular pattern from the site of initial colonization. 

The sheer complexity of the corrosion process in the presence of microoiganisms and microbial biofilms makes data interpretation difficult. Thus, whenever possible, multiple techniques should be used for assessing any microbial corrosion situation before conclusions are drawn. An outline of the laboratory techniques used most successfully in the past, along with any cautions for applying those techniques when the corrosion is influenced by microorganisms will be given below. 

Audouard JP, Compere C, Dowling NJE, Feron D, Festy D, MoUica A, Rogne T, Scotto V, Steinsmo U, Taxen K, Thierry D (1995) Effect of marine biofilms on high performance stainless steel exposed in European coastal waters. In Microbial corrosion proceedings of the 3rd international european federation of corrosion (EFC) Workshop European Federation of Corrosion EFC Publication no. 15, pp 198-210... 

Biofilms can promote corrosion of fouled metal surfaces in a variety of ways. This is referred to as microbiaHy influenced corrosion. Microbes act as biological catalysts promoting conventional corrosion mechanisms the simple, passive presence of the biological deposit prevents corrosion inhibitors from reaching and passivating the fouled surface microbial reactions can accelerate ongoing corrosion reactions and microbial by-products can be directly aggressive to the metal. 

Recently, there has developed a greater recognition of the complexity of the MIC process. MIC is rarely hnked to a unique mechanism or to a single species of microorganisms. At the present state of knowledge, it is widely accepted that the growth of different microbial species within adherent biofilms facihtates the development of structured consortia that may enhance the microbial effects on corrosion. 

MIC depends on the complex structure of corrosion products and passive films on metal surfaces as well as on the structure of the biofilm. Unfortunately, electrochemical methods have sometimes been used in complex electrolytes, such as microbiological culture media, where the characteristics and properties of passive films and MIC deposits are quite active and not fully understood. It must be kept in mind that microbial colonization of passive metals can drastically change their resistance to film breakdown by causing localized changes in the type, concentration, and thickness of anions, pH, oxygen gradients, and inhibitor levels at the metal surface during the course of a... 

N. J. E. Dowling, J. Guezennec, and D. C. White. Facilitation of corrosion of stainless steel exposed to aerobic seawater by microbial biofilms containing both facultative and absolute anaerobes. In Proceedings Volume. Inst Petrol Microbiol Comm Microbial Problems in the Offshore Oil Ind Int Conf (Aberdeen, Scotland, 4/15-4/17), 1986. 

H. A. Videla, P. S. Guiamet, O. R. Pardini, E. Echarte, D. Trujillo, and M. M. S. Freitas. Monitoring biofilms and MIC (microbially induced corrosion) in an oilfield water injection system. In Proceedings Volume. Annu NACE Corrosion Conf (Corrosion 91) (Cincinnati, OH, 3/11-3/15), 1991. 

Microbial activity is a major concern in systems in which water-based fluids are used. Particularly, glycol fluids provide a good source of nutrition to some types of biological species. In salt-based brines, however, microbes do not survive because of high osmotic pressure. When microbes start to grow inside a system, they create a layer known as biofilm on the walls of the pipes and heat exchangers. This reduces the heat transfer rate. Some microbes are capable of creating acids and hence cause a substantial amount of corrosion in the system. 

Biofilm samples and microbial contamination of drinking water flowing through the model in result of biofilm formation were analyses with standard methods (APHA, 1992). Number of heterotrophic bacteria (R2A/22°C/7d) coliforms and bacteria from different physiological groups, including corrosion related bacteria were determined. 

The increasing occurrence of microbial and nosocomial infection has stimulated research activities into antimicrobial polymers and textiles [19, 25, 34]. Most medical textiles and polymeric materials used in hospitals are conductive to crosstransmission of diseases, as most microorganisms can survive on these materials for hours to several months [17, 26]. Thus, it would be advantageous for polymeric surfaces and textile materials to exhibit antibacterial properties so as to reduce and prevent disease transmission and cross-contamination within and from hospitals. N-halamines exhibit a similar antimicrobial potency to chlorine bleach, one of the most widely used disinfectants, but they are much more stable, less corrosive and have a considerably reduced tendency to generate halogenated hydrocarbons, making them attractive candidates for the production of antimicrobial polymeric materials. N-halamine compounds are currently used as antimicrobial additives to produce polymers with antimicrobial and biofilm-limiting activities. 

The most aggressive corrosive attacks occur in the presence of microbial communities that contain a variety of bacteria. In these communities, the bacteria act cooperatively to produce favorable conditions for the growth of each species. For example, obligate anaerobic bacteria can thrive in aerobic environments when present beneath biofilms/deposits in which aerobic bacteria consume the oxygen. In the case of underground pipelines, the severe attack has been associated with acid-producing bacteria in such bacterial communities (Fig. 4.9). 

---